The lightweight nature of synthetic fiber rope provides many performance and economic advantages over metal wire rope. For example, when used in conjunction with sheaves in applications known as cyclic bend over sheave (CBOS) applications (e.g., cranes, elevators, heave compensation systems, and pulling lines), synthetic fiber rope allows for the use of equipment having a smaller footprint, less weight, and less power consumption than similar equipment for metal wire rope. However, regardless of whether metal wire or synthetic fiber rope is used in a particular application, it is critical to be able to assess the rope's condition to provide and maintain a retirement criteria to assure safe operation of the rope.
A crucial and dynamic component used for determining and maintaining an accurate retirement criteria is the structural integrity of the rope. The integrity of the rope is used in conjunction with other components including user preferences (e.g., replacement at 50% strength, etc.) and the particular application, (e.g., mooring, cranes and winches, safety lines, etc.) to determine the retirement criteria.
A number of SHM and NDE systems and methods that measure rope and cable structural integrity to determine the retirement criteria of steel and other metallic wire ropes have been developed. However, there are no generally accepted methods of measuring structural integrity for synthetic ropes, and thus the general practice for determining retirement criteria for synthetic fiber ropes relies on visual inspections and/or by tracking the history of usage for each rope. Visual inspections are inherently subjective and history-of-usage tracking can be highly inaccurate, therefore current retirement criteria are not based on meaningful parameters.
Responsive to the deficiencies of visual inspections and history of usage tracking, a number of objective methods for SHM and NDE for synthetic fiber ropes have been developed. The methods use secondary materials such as conductive carbon fibers and glass or polymeric optical fibers intertwined with the synthetic fibers of the rope. In theory, the secondary materials undergo the same stresses and wear as the synthetic fibers. The stresses on the secondary materials can be easily measured and from these measurements the stresses and wear on the synthetic fibers is inferred. In reality, because the materials are inherently different and due to the complicated structure of the rope, the synthetic fibers can undergo stresses and wear that the secondary materials do not. Furthermore, the secondary materials are only exposed to stresses in their immediate vicinity and, as such, there may be sections of the rope in which stresses and wear are not measured. For these reasons, measurements obtained using secondary materials directly reflect the integrity of the evaluation materials only and indirectly reflect that of the synthetic fibers that make up the rope itself. Furthermore, synthetic fiber ropes intertwined with secondary materials may be less strong, have different than expected abrasion properties, and be more difficult to manufacture than ropes formed entirely from synthetic fibers. For at least these reasons, methods for measuring the structural integrity of rope with optical fibers are not widely practiced.
Another method of monitoring and evaluating synthetic fiber ropes involves longitudinal waves propagated over a length of rope. Such a method may utilize longitudinal wave propagation theory such as described by M. Ferreira et al. in “Non Destructive Testing of Polyaramide Cables by Longitudinal Wave Propagation: Study of the Dynamic Modulus”, Polymer Engineering and Science, Vol 40, No. 7, July 2000. This method calls for at least some physical contact with the rope as probes or tappers directly contact the rope to introduce the acoustic signal. Furthermore, to determine meaningful retirement criteria with data from longitudinal waves, the rope must be held under constant tension (e.g., elevator cables, antenna stays, etc.) when being evaluated. This limits the use of this method to applications monitoring a stationary length of rope. Even then, vibrations in the rope introduced by devices in the rope's environment, such as motors and sheaves, are picked up by the transducers, resulting in distorted wave measurements. A further deficiency of this method is that it measures the entire length of the rope between the transducer and receiver, which are normally placed at each end of the rope. Therefore the resolution of this system is limited by the distance between the transducer and receiver. If only a small part of the rope is damaged, the system would not be able to indicate the specific location of the damage and the entire rope would have to be considered suspect. Another limitation to this method is that it cannot be used in situations where a portion of the rope is inaccessible, such as a rope used in conjunction with an offshore crane where one end could be under thousands of feet of water.
Therefore, there is a need for a system that enables structural health monitoring of synthetic fiber ropes in a way that does not need to have direct contact with the rope, access to two distinct points along the rope, or the use of secondary materials, with greater resolution, and furthermore, is able to be used on a fast moving rope, and with a variety of rope fibers and construction methods.